The present invention relates to reduction of distortion caused by an amplifier in general and in particular to linearisation, by means of adaptive predistortion, of an amplifier which gives rise to distortion that varies within the range of frequencies in which the amplifier works.
Distortion is an undesirable change in a signal that may arise for instance when the signal is amplified by an amplifier with some kind of non-linearity that distorts the signal. Non-linearity in an amplifier can particularly become a problem when the amplifier is set to work close to its maximum voltage.
Distortion can cause the signal to be misinterpreted. A system that is particularly sensitive to distortion is a multicarrier telecommunications system. In a multicarrier system information is transmitted on a common medium in channels separated by the assignment of a portion of the available frequency spectrum to each individual channel. Harmonics caused by distortion may often appear outside the working bandwidth of a system and can thus easily be disregarded or removed by filtering. Harmonics caused by distortion in a multicarrier system are often difficult to detect and compensate for since a harmonic to one channel may interfere with another channel. Multicarrier systems therefore require highly linear amplifiers. Imperfections in the linearity of an amplifier can however be improved using a number of different techniques, such as for instance feedback, feedforward and predistortion.
Another type of system that is particularly sensitive to distortion is a telecommunications system with echo cancellation. Echo cancellation makes use of a linear filter to cancel echo and this filter will not be able to cancel the echo if it includes distortion.
With feedback linearisation a portion of the output of the amplifier is returned and subtracted from the input. This reduces gain and distortion and increases linearity.
Feedforward linearisation is possible when the actual distortion caused by the amplifier is known or can somehow be extracted or measured. It is then possible to combine the output of the amplifier with a signal that cancels the distortion in the amplified signal.
With predistortion linearisation, distortion is intentionally added to the input of the amplifier in such a way that the added distortion effectively cancels the distortion generated by the amplifier itself. There are many available predistortion techniques to choose from.
The U.S. Pat. No. 5,606,286 describes a predistortion technique in which a radio frequency signal is divided in three different parts which are exposed to different amounts of distortion and then recombined to form a single predistortion signal.
In the U.S. Pat. No. 4,453,133 predistortion components are developed by an amplifying device similar to a power amplifier, the linearity of which it is desirable to improve.
An adaptive predistorter created from a Wiener system model of an HPA (high power amplifier) and a filter is shown in Hyun Woo Kang et al, xe2x80x9cOn Compensating Non-linear Distortion of an OFDM System Using an Efficient Adaptive Predistorterxe2x80x9d, IEEE Transactions on Communications, Vol. 47, No.4, pp. 522-526; April 1999.
Digital predistortion techniques using look-up tables (LUT) are described in E. Jeckeln, F. Ghannouchi and M. Sawan, xe2x80x9cLinearization of Microwave Emitters using an Adaptive Digital Predistorterxe2x80x9d and in J. de Mingo, A. Valdovinos and J. Ruiz, xe2x80x9cPerformance of a Digital Base-band Predistorting Amplifier Linearizer Implemented in Fixed Point DSPsxe2x80x9d.
A linedriver is an amplifier that is used in digital subscriber line (DSL) telecommunications transceivers to drive electrical signals onto a telecommunications line. Non-linear distortion in the linedriver is a common limiting factor of the performance of the transceiver. The linedriver in an ADSL-system may be required to work at frequencies in the range of 0 Hz -1 MHz, which means that the linedriver, in addition to high linearity, is required to have a bandwidth that is in the same order of magnitude as its working frequency. It is further desirable to limit the power consumption in the linedriver. However it is hard to achieve both low power consumption and wide bandwidth for an amplifier. In addition the non-linearity of an amplifier tend to increase the closer to saturation the amplifier has to work. In view of linearity it is thus desirable to have an amplifier with high power consumption working far below saturation as a linedriver, while in view of power efficiency it is desirable to have an amplifier with low power consumption working close to saturation.
Typically a phase compensated amplifier with feedback is used as a linedriver in xDSL-systems. Phase compensation is a method to ensure stability in the amplifier. An amplifier with phase compensation shows high open loop gain for low frequencies but the gain decreases proportionally to the frequency. Feedback will, as mentioned above, reduce gain and distortion and increase linearity. It is possible to attenuate distortion satisfactorily at low frequencies by means of the feedback, but due to the fact that the open loop gain decreases proportionally to the frequency the effect of the feedback on distortion will also decrease with the frequency. A phase compensated amplifier with feedback will thus show distortion that depends on and increases with the frequency.
The present invention solves the problem of effectively attenuating distortion introduced by an amplifier, which distortion is characterised in that it depends on and increases with the frequency. As mentioned above one type of amplifiers that introduce frequency dependent distortion is a phase compensated linedriver with feedback. The invention thus solves the problem of reducing distortion introduced by said type of linedriver or by an amplifier with similar distortion characteristics. Several prior art techniques for increasing linearity and reducing distortion caused by amplifiers have been mentioned above. None of these prior art techniques are however suitable or specially adapted for dealing with frequency dependent distortion. Frequency dependent distortion usually only becomes a problem when the required bandwidth of the amplifier is in the same order of magnitude as its working frequency. The range of the frequency spectrum that the amplifier has to cover can then be said to be wide compared to the frequencies of the signals to be amplified. When s aid range is narrow compared to the order of frequencies of the signals to be amplified the distortion usually has substantially the same order of magnitude within the entire range. This is usually the case in radio frequency (RF) applications. A RF amplifier works at high frequencies but covers a comparably small range of the frequency spectra. For most RF application it is therefore not necessary to take frequency dependency of the distortion in consideration.
An object of the present invention is to provide a predistorter for predistorting an input signal of an amplifier, which amplifier introduces frequency dependent distortion as mentioned above.
Another object of the present invention is to provide a method for predistorting an input signal of an amplifier, which amplifier introduces frequency dependent distortion as mentioned above.
A further object of the present invention is to provide an amplifier with high linearity which amplifier includes a predistorter that reduces frequency dependent distortion in the amplifier""s output.
The present invention solves the above stated problem by means of a predistortion technique involving high pass filtering such that the distortion compensation from predistortion is set to work for the specific frequencies for which further distortion compensation is required.
According to one aspect of the invention the above stated problem is solved by means of a predistorter for predistorting an input signal of an amplifier, which predistorter produces an output signal based on a combination of a first signal component and a second signal component, which first signal component is linearly dependent on an input signal to the predistorter and which second signal component depends on a high pass filtered non-linearity dependent on the input signal to the predistorter.
According to another aspect of the invention the above stated problem is solved by means of a method for predistorting an input signal of an amplifier, which method includes the steps of producing a first signal component based on an input signal to the predistorter, producing a second signal component by high pass filtering a non-linearity dependent on the input signal to the predistorter and combining the signal components to a predistorted input signal of the amplifier.
An advantage with the present invention is that it allows the performance of an amplifier to be improved by reducing distortion and increasing linearity.
Another advantage is that the invention allows low power amplifiers with relaxed distortion requirements to be used as for example linedrivers in many telecommunications systems. In systems where the invention allows a more power consuming high quality amplifier to be replaced by a cheaper amplifier with less power consumption costs can be reduced.
Yet another advantage of the present invention is that it is fairly simple. It does not require complex calculations to be performed.
A further advantage of an embodiment of the present invention is that the inventive predistortion can adapt to changes over time in the characteristics of the amplifier.
The present invention will now be described in more detail by means of preferred embodiments and with reference to the accompanying drawings.